I have a copy of John Ashbery's book Chinese Whispers and a copy of Drawing a Blank by Daniel Ehrenhaft both signed by the author. I'd like to sell these books but I'm not sure where to find out how the signature would change pricing (I'd hope they'd be worth more now!). I also haven't found a selling site to indicate the presence of a signature. I checked ebay, amazon.com and half.com and did not see any place to indicate that the books I wanted to sell were signed. Any suggestions?